Semi-permeable hollow fiber membranes are known in detail in, for example, EP-A-0 568 045, EP-A-0 168 783, EP-B-0 082 433, WO 86/00028, and EP 0 824 960. These membranes are manufactured from polymeric synthetic materials, they have asymmetric structure with high diffusive permeability (clearance) and have water filtration capability with ultrafiltration in the range of low flux to high flux. In EP-A-0 305 787, a 3-layer structure membrane and filter with corresponding performance, is disclosed.
The membranes according to prior art are well performing, but still have some space for improvement and optimization.
One limiting feature of these membranes are that the fluid to be filtered is intended to flow in the interior lumen of the hollow fiber and the filtrate to go through the fiber wall from the lumen side to the outer wall side. In order not to have these filters to foul or clog, the dimensions of the hollow fiber, such as inner diameter, wall thickness and so on, have to be big enough to allow a good and high flow within the hollow fiber lumen.
In DE 199 13 416 it is suggested to make the filtration from outside to inside, i.e., to have the selective layer on the outside.
However, when working with body fluids like blood it is of most importance that the membrane surface, which is intended to be brought in contact with the body fluid, is as smooth as possible, has low protein adsorption, high biocompatibility, and a low thrombogenicity.